1. Field of the Invention
This invention relates to the determination of the periodicity of a characteristic of a textile fabric which changes in a repeating fashion along the length of the fabric. Information relating to such a characteristic can be used to control fabric treatment processes, such as stentering or compacting, for example, to adjust the number of courses of the fabric per unit length to a desired value.
The term "characteristic" is taken to refer to any feature of the fabric (irrespective of whether the fabric is knitted or woven) which repeats itself. For example the feature may be a structural feature such as the courses or warp threads of the fabric, or a repeating pattern such as stitches, holes (in lace for example), dyed regions or different coloured threads, or some other structural feature of the fabric. For simplicity, the invention will be described with reference to counting the courses of a fabric but the present invention is intended to cover other uses, where the context fits.
The present invention is particularly useful for counting the courses of a fabric. The course count of a woven fabric is the number of picks in a unit length of the fabric, and the course count of a knitted fabric is the number of courses in a unit length of the fabric.
2. Description of Prior Art
In some processes for the treatment of fabrics, such as stentering used to stretch or shrink the fabric or compacting (used to shrink the fabric) to obtain uniformity of spacing of the courses and wales (or warp and weft threads) it is necessary to adjust the process parameters to compensate for variations in the fabric entering the process equipment.
In a stenter, the fabric is stretched or overfed as it is passed through a heating zone on pin chains. This is usually achieved by controlling the speed of rotation of rollers over which the fabric passes as it enters and leaves the heating zone and the fabric is held taut on the pin chains to achieve uniform density.
In both of these processes, it is very difficult to count all the courses accurately and to use this count to control the rate of feed of the fabric into, and out of, the stenter or compactor. Not only are the courses irregularly spaced but the fabric may also be puckered, folded, loose or taut.
A previous method of determining course count has employed a photoelectric cell to measure the transparency of the fabric along its length and means for measuring the peak amplitudes of the response curve of the cell thus obtained. Such curves are very irregular in shape and for many purposes, the reliability of the period measurements obtained in this way is not sufficiently high.
In the case of a course count determination, a typical signal will comprise one or more periodic components directly related to the course count, together with other random and/or repetitive components which may completely mask the course count component, at intervals.
The basic course count frequency may be difficult to measure by this known method of counting the signal peaks above a chosen threshold level since:
(a) peaks above the threshold level may be generated by a combination of unrepresentative signals and the basic signal, and PA1 (b) the basic signal may not be present all the time (for example, because the expected apertures in the fabric are obscured). PA1 (a) sensing a property related to the said characteristic at pairs of positions which are spaced apart by a distance, S, along a length of the fabric, PA1 (b) generating signals representative of the magnitude of the said property at the said positions, PA1 (c) summing the products of the signals generated at each pair of positions in accordance with the formula: EQU .fwdarw.x.sub.(y) .multidot.r.sub.(y+S) PA1 (d) repeating steps (a) to (c) for different dimensions, S, and PA1 (e) determining the value of S at which the summation of the signals is a maximum and using this value of S to generate an output signal representing the value of the periodicity. PA1 x.sub.(y+S) is a function x representing the composite signal at a position y+S along the fabric PA1 Y=length along the fabric PA1 S=an interval of length along the fabric.
Furthermore the maximum accuracy of periodicity measurement by this method is determined by the number of courses counted in any one sample length.
It is an object of the present invention to improve the reliability of determinations of the periodicity of a characteristic of a textile fabric such as its course count.